1. Field of Invention
This invention relates to an electrical wire having a halogen-free resin composition as an electrically insulating covering on an electrical conductor core.
2. Description of Related Art
Polyvinyl chloride has been much used as the covering material of electrical wire for automobiles, because it is superior in properties such as mechanical strength, extrusion processability, flexibility and coloring property. However, with recent concern for the global environment, halogen-free resin materials have come to be used in the production of automobile parts, including the covering of electrical wires used in automobiles, in place of polyvinyl chloride, because polyvinyl chloride discharges a harmful halogen gas on combustion.
A halogen-free resin composition, in which a metal hydroxide is blended with a polyolefin-based polymer as a flame-retardant, is known as a wear resistant resin composition that does not generate poisonous gases such as halogen gases upon combustion. See, for example, JP-A-7-176219, JP-A-7-78518. So that the flame-retardant resin composition has a self-extinction property, a large quantity of a filler such as a metal hydroxide must be added. However, addition of large quantities of metal hydroxide to the composition causes problems in mechanical strength such as reduced wear resistance, tensile strength, and the like. Also, as the amount of metal hydroxide is increased, water resistance deteriorates and, in particular, electrical properties upon immersion in water deteriorate. Furthermore, when the amount of filler in a resin composition is increased, the composition tends to whiten, particularly upon bending.
In order to prevent a reduction in mechanical strength in a resin composition, it is possible to increase the amounts of a polypropylene having a comparatively high hardness, and a high density polyethylene. However, the flexibility of an electrical wire covered by such a composition and the processability of the composition are reduced.
In order to enhance flexibility of a resin composition, a resin having a low crystalline component content, i.e. having low crystallinity, and a rubber component can be employed. However, such compositions have reduced oil resistance. Alternatively, to enhance flexibility, an elastomer having a low melting point and softening point could be used. Such compositions, however, are deformed at high temperatures and are subject to melt-fusing.
Accordingly, an object of this invention is to provide an electrical wire having a halogen-free, olefin-based resin as its covering. A further object of this invention is to provide a good balance of properties that are desired in covering materials of electrical wire used, for example, in an automobile. Such properties include water resistance, oil resistance, flexibility at high temperatures, wear resistance, minimal whitening, and the like.
According to this invention there is provided an electrical wire including a conductor and an electrically insulating covering on the conductor. The covering is a resin composition including at least one first polymer, at least one second polymer, and filler particles. The first and second polymers possess at least one of the following features (a), (b) and (c):
(a) the first and second polymers each have an immersion saturated water absorption rate of about 3% or less;
(b) the first polymer has a crystallinity of less than about 50% and the second polymer has a crystallinity of about 50% or more; and
(c) the first polymer has a melting point or softening temperature of less than about 100xc2x0 C. and the second polymer has a melting point or a softening temperature of at least about 100xc2x0 C.
In various exemplary embodiments, the first and second polymers possess at least two of the features (a), (b) and (c). In various other exemplary embodiments, the first and second polymers possess all three of the features (a), (b) and (c).
Preferably there are no polymer resin components in the resin composition, other than the first and second polymers. Thus, for example, when only a single first polymer and a single second polymer are employed, the first polymer and the second polymer are the only two polymer components present.
According to this invention, there is also provided a method of making electrical wire by mixing the above-described polymer components and filler particles, and applying the resulting resin composition to an electrical conductor as a covering.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of this invention.